Radicals Produced in Single Crystals of Thymine Derivatives by UV and X-Rays

Single crystals o f anhydrous thymine (T), thymine monohydrate (T ■ H 20 ) , 5,6-dihydrothymine (TH 2), 1-methylthymine (mT) and thymidine (dT) were irradiated with X-rays and UV between 77 K and 300 K. Six types o f radicals were analyzed by ESR-spectroscopy at 9.5 GHz and 35 GHz after exposure to X-rays. The anion radical occurred only in T • H 20 at 77 K, the 4-yl radical only in TH 2 at 77 K and the 1-yl radical only in T between 77 K and 300 K. The 6 -yl radical was found in T, TH 2 and mT. It was converted into the 5-yl radical irreversibly by heat or white light (A < 600 nm). The 5-yl radical appeared in all compounds at room temperature. The highest thermal stability was found for the 7-yl radical which was present at room temperature in all compounds except TH 2 . UV-irradiation (A = 320 nm) produced radicals only in three crystals (T, TH 2, dT). In T the 5-yl radical was found after exposure at 300 K, and two other radicals, 1-yl and 7-yl, at 77 K. Also at 77 K, the 7-yl radical was present in dT and the 5-yl radical in TH 2 and dT.